Traditionally, heating, ventilating and air conditioning (HVAC) systems have been designed to mix heated or cooled air, for thermal loads, with outside air, for ventilation, at an air handling or processing unit. Mixed air is then delivered in a common duct system to the spaces to be conditioned. As used herein, it will be understood that the expressions "conditioned" and "conditioning" shall include any one or more of heating, cooling, ventilating or filtering and recycling air; and the expressions "ventilated" and "ventilation" shall include air which is taken into an HVAC system from outside the structure, as well as air which is returned from a room in the structure and filtered to remove contaminants, and mixtures of outside air and filtered return air. The addition of ventilation air to the supply air of a HVAC system is designed to prevent endless recycling of unfiltered system air and the attendant build up of undesirable air-born containments. In some urban environments, of course, it is not clear that the outside air is "fresh" or even as good as the returned supply air, nevertheless, the addition of ventilation air generally is believed to be highly desirable.
At the present time, the flow rate of ventilation air to be added to HVAC system supply air is often prescribed by ASHRAE Standard 62-1989. The ASHRAE Standard is set by American Society of Heating, Refrigeration and Air Conditioning Engineers, and it has been adopted by code in many states. Even when the ASHRAE Standard is not required by code, it is usually the industry standard. For offices, the present ASHRAE Standard for the flow of ventilation (outside and/or filtered) air into a room or office is a minimum of 20 cubic feet per minute (cfm) per person.
Thermal loads, however, determine the amount and temperature of the conditioned or supply air which must be used in a space to achieve the desired conditioning effects. Thermal loads in office spaces are usually determined by sensing the temperature in the room, and there can be little correlation between the thermal load and occupancy of a space in a modern office building. Thus, factors such as lighting, computer equipment and other heat sources can produce considerable variation of the thermal load from office to office independently of occupancy.
One of the most common HVAC systems employed in modern office buildings is the variable-air-volume (VAV) conditioning system. Such systems vary the volume of supply air discharged into a room in response to the thermal load demand, as determined by sensing the room air temperature. VAV systems offer a number of potential operating and cost advantages as compared to constant volume, variable temperature systems. As will be appreciated, however, if the ventilation air flow rate is prescribed by occupancy, and the thermal demand is not an absolute function of occupancy, the standard approach of simply adding ventilation air to the supply air will not provide offices with sufficient ventilation air when thermal loads are low. Thus, when the thermal load in an office is relatively low, the VAV air diffuser will close down and deliver less supply air, or even no supply air, to the office. Nevertheless, the office may have several occupants, and the quantity of air being discharged out of the VAV diffuser will not include sufficient ventilation air to meet the ASHRAE 62-1989 Standard, or other minimum ventilation standards or regulations.
One approach to this problem has been to increase the amount of ventilation air added to the supply air so that even under the lowest thermal loads, sufficient outside air will be included in the air discharged from the VAV device. The problem with this approach is that it requires conditioning of a much higher volume of ventilation air, with attendant increased costs. Another approach has been to add sufficient ventilation air to the central conditioning unit to meet the ASHRAE Standard, on average and simply disregard the fact that all spaces are not adequately ventilated. There is a liability exposure in such an approach when the problem of a "sick" buildings occurs. Thus, if health problems do arise in the building, and it is shown that many rooms fall below the ASHRAE Standard or other legal minimum ventilation standard, the addition of sufficient ventilation air to the system "on average" is not likely to be an acceptable solution nor an approach to avoiding liability.
A third prior art approach to adequate ventilation is to essentially duplicate the HVAC system with a parallel ventilation air system. Thus, a ventilation air treatment unit and blower, with separate ducts to each office, and separate ventilation air diffusers in each office are installed. This approach, however, creates an undesirable duplication of diffusers in each office, which can be unsightly as well as add extra expense.
VAV conditioning systems typically include a room air temperature sensing apparatus located in many, and often each, of the spaces which are conditioned. The room air temperature sensor can be located in a position which is remote from the supply air diffuser, or it can be located in the diffuser itself. One technique that is commonly employed in VAV systems, in order to ensure room air flow past the room air temperature sensing device, is to positively induce the flow of room air past the temperature sensing device. This is usually done by the discharge of supply air from the diffuser. Thus, a nozzle or orifice can be positioned for the discharge of a small volume of supply air from the diffuser, even when the diffuser is closed, so as to induce the flow of room air past the room air temperature sensor. This ensures that the room air temperature sensor is not sensing air temperature under stagnant conditions, and thus that the room air temperature sensor is more accurately measures average room temperature.
The discharge of a small volume of supply air to induce room air flow past temperature sensors has been used for many years in connection with thermally-powered VAV air diffusers. U.S. Pat. Nos. 4,509,678, 4,537,347 and 4,821,955, for example, all describe VAV diffusers which are thermally powered and include induction air discharge arrangements in which supply air is discharged into the room even when the diffuser is "closed" so as to induce room air flow past the temperature sensor mounted in the diffuser. The temperature sensors themselves are combination sensor-actuators which respond to temperature changes to produce displacement of VAV control vanes, dampers or disks through linkage assemblies in order to open and close the diffuser as the thermal load varies.
Accordingly, it is an object of the present invention to provide a VAV diffuser apparatus and method capable of meeting the ASHRAE 62-1989 Standard for ventilation, or other local ventilation standard, while still being highly efficient and capable of accommodating the conditioning of spaces having thermal loads which vary considerably.
Another object of the present invention is to provide a VAV diffuser system which is capable of discharging ventilation air into a space at a rate which is independent of, or decoupled from, the thermal load.
Still a further object of the present invention is to provide a thermally-powered diffuser which is capable of discharging ventilation air into a space at a rate sufficient to meet the ASHRAE 62-1989 Standard, or other local ventilation standards, under essentially thermal no-load conditions.
Another object of the present invention is to provide a method or process of ensuring the flow of sufficient ventilation air into a space being conditioned by VAV diffuser system so that thermal load variations do not reduce ventilation air flow below a desired threshold.
Still a further object of the present invention is to provide a VAV diffuser apparatus and method which is efficient to operate, suitable for retrofitting to existing VAV systems, and is inexpensive to construct, install and maintain.
The variable-air-volume diffuser system and method of the present invention have other objects and features of advantage which will be set forth in more detail in, and will be apparent from, the following Best Mode of Carrying Out the Invention and accompanying drawings.